An optical pickup apparatus used for reading or writing information from or on an information layer of an optical disc medium includes a light intensity control device for monitoring a part of a laser beam output from a light source and controlling the light intensity thereof, such that the laser beam can be directed to the information layer of the optical disc medium at an appropriate light intensity.
FIG. 8 shows an optical pickup apparatus 400 disclosed in Patent Document 1. Referring to FIG. 8, an information medium 109 is a DVD (Digital Versatile Disc) or a CD (Compact Disc), and the optical pickup apparatus 400 records or reproduces information on or from the information medium 109.
The optical pickup apparatus 400 includes a DVD light source 101 for outputting a light beam 121 for DVDs having a wavelength of 650 nm, a CD light source 102 for outputting a light beam 122 for CDs having a wavelength of 780 nm, a beam splitter 103, a ½ waveplate 104, a polarizing beam splitter 105, a ¼ waveplate 106, a collimator lens 107, an objective lens 108, a sensor lens 110, a light detection section 111, and a light receiving element 112.
The beam splitter 103 transmits the entirety of a P-wave component and the entirety of an S-wave component of the light beam 121 for DVDs, and reflects the entirety of a P-wave component and the entirety of an S-wave component of the light beam 122 for CDs. The polarizing beam splitter 105 transmits the P-wave components of the light beams and reflects the S-wave components of the light beams. The light detection section 111 detects a tracking error signal, an information signal or the like from light reflected by the information medium 109. The light receiving element 112 outputs an electric signal to be fedback to a light source driving circuit (not shown) in accordance with the light intensity, and is referred to as the “front monitor”.
An optical path for the light beam 122 for CDs is drawn as being offset from an optical path for the light beam 121 for DVDs, but is actually the same. The ½ waveplate 104 is located so as to receive light outside an effective luminous flux of the light beam 121 for DVDs and the light beam 122 for CDs.
Now, an operation of the optical pickup apparatus will be described. The light beam 121 for DVDs which is output by the light source 101 is transmitted through the beam splitter 103. Only the P-wave component of the effective luminous flux of the light beam 121 is selected by the polarizing beam splitter 105 and converted into circular polarization by the ¼ waveplate 106. The light beam 121 transmitted through the ¼ waveplate 106 passes through the collimator lens 107 and the objective lens 108, and then forms a light spot on the information layer of the DVD disc 109. The light reflected by the information layer is transmitted through the objective lens 108 and the collimator lens 107, is converted back into the P-wave component by the ¼ waveplate 106, is reflected by the polarizing beam splitter 105, and is guided to the sensor lens 110. The reflected light which is passed through the sensor lens 110 is received by the light detection section 111.
The light beam 122 for CDs which is output by the light source 102 is totally reflected by the beam splitter to be a light beam of the P-wave component. Like the light beam 121, the effective luminous flux of the light beam passes through the polarizing beam splitter 105, the ¼ waveplate 106, the collimator lens 107 and the objective lens 108, and then forms a light spot on the information layer of the CD disc 109. The light reflected by the information layer passes through the objective lens 108, the collimator lens 107, the ¼ waveplate 106, the polarizing beam splitter 105 and the sensor lens 110, and is received by the light detection section 111.
By contrast, the light outside the effective luminous flux of the light beam 121 for DVDs passes through the ½ waveplate 104 to be polarized to S-wave component light, is totally reflected by the polarizing beam splitter 105, and is received by the light receiving element 112. The light receiving element 112 outputs an electric signal in accordance with the light amount of the received light, and feeds the electric signal back to the light source driving circuit (not shown).
The light outside the effective luminous flux of the light beam 122 for CDs also passes through the ½ waveplate 104 to be polarized to S-wave component light, is totally reflected by the polarizing beam splitter 105, and is received by the light receiving element 112. The light receiving element 112 outputs an electric signal in accordance with the light amount of the received light, and feeds the electric signal back to the light source driving circuit.
As described above, the optical pickup apparatus 400 including the light sources 101 and 102 for independently outputting light beams of different wavelengths can change the polarization direction of the light beams independently. Therefore, the intensity of the light beam 121 for DVDs and the intensity of the light beam 122 for CDs can be controlled independently.
However, as recordable DVDs have become common and a higher recording speed of the optical disc apparatus has been demanded recently, the optical disc apparatus are desired to be stable against a temperature change. Also, the optical disc apparatus is strongly desired to be reduced in size. It has been proposed to adopt a single light source for outputting a plurality of types of light beams having different wavelengths in order to reduce the size of the optical disc apparatus.
However, where a single light source which accommodates an active layer for outputting a light beam for DVDs and an active layer for outputting a light beam for CDs in one package is adopted, a problem occurs that the polarization direction of each light beam cannot be independently changed unlike in the above-described structure. In addition, there is another problem that a temperature change, caused by the output of the light beam, changes the polarization ratio and the polarization angle of the light beams. As a result, the intensity of the light beam directed to the optical disc medium is changed. In order to solve the inability to freely change the polarization direction and prevent the intensity of the light beam from being changed, Patent Document 2 discloses an optical pickup apparatus which uses a polarization separation element for separating light intensity-controlling light (hereinafter, referred to as “monitoring light”) into a P-wave component and an S-wave component and controls the light intensity for the light source with the P-wave component.
FIG. 9 shows a conventional optical pickup apparatus disclosed in Patent Document 2. A semiconductor laser element 201 includes two active layers 201a and 201b for outputting two light beams having different wavelengths. The first active layer 201a outputs a light beam 221 having a wavelength of 650 nm for DVDs (hereinafter, referred to as the “DVD wavelength”), and the second active layer 201b outputs a light beam 222 having a wavelength of 780 nm for CDs (hereinafter, referred to as the “CD wavelength”).
The light beam 221 or 222 which is output from the semiconductor laser element 201 reaches a polarizing beam splitter (hereinafter, referred to as the “PBS”) 206 as parallel light via a collimator lens 204. The PBS 206 transmits a majority (e.g., 90%) of the P-wave component of the light beam incident thereon, and reflects a part thereof (e.g., 10%) by a polarization separation face 206a. The PBS 206 also transmits a part (e.g., 10%) of the S-wave component of the light beam incident thereon, and transmits a majority thereof (e.g., 90%).
The light beam which is passed through the PBS 206 passes through a ¼ waveplate 209 to be converted into circular polarization, reaches an optical disc 211 via an objective lens 210, and is reflected by a recording face thereof. The light reflected by the recording face of the optical disc 211 returns to the PBS 206 via the objective lens 210 and the ¼ waveplate 209. The ¼ waveplate 209 converts the circular polarization of the reflected light beam into linear polarization. The PBS 206 reflects the light beam on the return path by the polarization separation face 206a, and the reflected light beam reaches a light receiving face of a light detector 213 via a light collection lens 205 and a multi-lens 212.
In a direction in which the light reflected by the PBS 206 is directed, another PBS 207 is provided. The PBS 207 is located in a direction in which the light beam incident from the collimator lens 204 and reflected by the PBS 206 is directed. On the PBS 207, such reflected light beam for front monitoring is incident. The PBS 207 passes approximately 100% of the P-wave component of the incident light beam, and reflects approximately 100% of the S-wave component of the incident light by a polarization separation face 207a. Almost 0% of the P-wave component is reflected by the PBS 207, and approximately 100% of the S-wave component passes through the PBS 207. In a direction in which the light passed through the PBS 207 is directed, a front monitor 208 is provided. Only the P-polarization component, which is passed through the PBS 207, is received by the front monitor 208. Thus, laser output power control is performed. In an optical pickup 500 using a light source accommodating two active layers for respectively outputting a light beam for DVDs and a light beam for CDs in one package, the P-wave component is used as the monitoring light for controlling the light intensity for the light source.
By adopting the P-wave component of the light beam output by the semiconductor laser element 201 as the monitoring light, the intensity of the light output by the semiconductor laser element 201 can be stably controlled.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-110897
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-109773